Spurs have kept four clean sheets in their first 10 Premier League matches this season, and Argentina centre-back Romero is one of the six nominees for the Premier League's player of the month award for October.
Romero has formed a solid centre-back partnership with summer signing Micky van de Ven, with that pairing regularly starting alongside full-backs Pedro Porro and Destiny Udogie and in front of another summer addition - goalkeeper Guglielmo Vicario.
Postecoglou said: "The way our defensive side of the game has come together... the one constant and the rock in that [much-changed defensive unit] has been Romero, with the way he's taken on that responsibility of guiding those other guys around him so that they look assured.
"I think they feel security having him beside them, he's a big part of why we've done so well defensively.
"And there's more to come from him. He wants to improve and he works hard every day. He's a winner - you can see that in him with the way he trains and the way he plays, and that helps from a cultural perspective as well."